Pretensioned spring



NOV. 4, 1952 c KOEHLER 2,616,993

PRETENSIONED SPRING Filed July 17, 1947 2 SHEETS,SHEET 1 FIG. I

D. C. KOEHLER ATTORNEY Nov. 4, 1952 D. c. KOEHLER 2,616,993

PREITENSIONED SPRING Filed July 17, 1947 2 SHEETS-SI-IEET 2 FIG. .5

APPROXIMATION TO ELASTIC CURVE USING TWO SHARP BENDS IDEAL ELASTIC CURVEl 3 y: .294 3 x x CLAMP/NC POINT CENTERL/NE or conmcrs I 2 .500} .750L000 Lao 1.500 L750 2.017

FIG 6 /N VEN TOR By 0. c. KOEHL 5/;

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ATTORNEY Patented Nov. 4, 1952 PRETENSIONED SPRING Donald C. Koehler,Maplewocd, N. J assignor to Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York Application July 17, 1947,Serial No. 761,617

10 Claims.

This invention relates to elongated spring members and to methods forpretensioning said members, and more particularly to springs forelectrical switching apparatus and new and improved methods forproducing a predetermined tension in such springs.

In various utilizations of leaf or wire spring members, it is often arequirement that such springs be arranged to exert a. certain amount ofpressure at a given degree of fiexure, i. e., that such springs shall bepreformed so as to exert a certain pressure upon subsequentpretensioning. Such requirements are found in applications to electricalapparatus, and particularly in applications to electrical switchingapparatus in which various electrical circuits are opened or closedunder the control of electromagnetic devices or relays. It is frequentlydesirable or necessary that the contact springs employed in such relaysbe pretensioned so as to exert a given pressure when in position in therelay, or, in some instances, so as to exert a required contact pressurewhen operated by an armature which may, itself, be supported or biasedby a pretensioned spring member.

A further requirement arising from the conventional arrangement of aplurality of contact springs mounted and arranged in parallelism is thatwhen the contact end of an individual spring is offset from the plane ofthe clamping surface by an amount sufficient to give the proper springtension, the shape of the curve must be controlled so that when thespring is mounted in a. relay it will lie approximately straight so asto maintain necessary clearances between adjacent springs. Suchrequirement particularly exists where adjacent springs are tensioned inopposite directions.

The pretensioning of contact or similar springs has generally beenaccomplished by the handforming of the member in a smooth curvethroughout its length. Attempts to preform such springs by a machineoperation have generally proved unsatisfactory inasmuch as minorvariations in spring thickness, hardness or elastic limit resulted in anon-uniform product which required further expensive and time-consumingmanual adjustment after installation in the relay.

As far as the applicant is aware, previous attempts in the art topretension springs have failed to meet all phases of the three-foldproblem, i. e., if a uniform accurate preformin of the spring wereobtained, the method either failed to obviate the necessity for anexpensive hand operation, or else, by placin but a single bend in thespring, necessitated having all springs tensioned in the same directionto attain the required clearances between adjacent members whilemaintaining relay compactness.

An object of this invention, therefore, is to accurately, uniformly, andeconomically preform spring members to have a predetermined tension whenin place.

Another object of this invention is to pretension resilient members sothat when under the required degree of tension, they will lie inapproximately a straight line.

A further object is to provide a simple and effective method formechanically producing a predetermined tension in such spring members.

In accordance with the general features of the invention, elongatedspring members are preformed to obtain a uniform predetermined degree ofpretensioning by placing a series of sharp transverse bends in thespring at various points along its length. The bends are so located andof such degree that the resulting member is formed to closelyapproximate the shape of a desired curve to provide a given degree ofsubsequent pretensioning. Subsequent pretensioning shall hereinafter beunderstood to mean that when said spring is flexed into an approximatelystraight line it will exert a reactive pressure of a predeterminedamount.

The invention may be more fully understood by reference to theaccompanying drawings in which:

Fig. 1 is an elevational view of a relay equipped with contact springsembodying the features of the invention;

Fig. 2 is an end view of the relay disclosed in Fig. 1;

Fig. 3 is a top view of a contact spring preformed in accordance withthe invention;

Fig. 4 is a side view of the contact spring shown in Fig. 3;

Fig. 5 represents a contact spring preformed to approximate a desiredcurve; and

Fig. 6 is an elevational view of a punch and die member which may beused to preform springs in accordance with the invention.

In order to demonstrate the employment of contact or support springspreformed in accordance with the present invention, an electromagneticdevice or relay has been depicted in Figs. 1 and 2 which is morecompletely described in Patent 2,472,709, granted June 7, 1949, to H. M.Knapp. The relay comprises a core I having an actuating coil 2, and anarmature 3. A plurality of contact springs 5, .6 and 1 are mounted in aspring pileup assembly I and extend in approximate parallelism to eachother and to the core I. The fixed or passive contact springs arsengagedby notches I I in fixed card I2, and are so preformed that wheninstalled in the relay they tend to bend upwardly at the contact end andthus exert-e, predeterminable pressure against the upper edges of thenotches II in spo-olhead I2. The advantage of such pretensioning of thefixed springs 5 and the employment of wide notches II is that thediiliculty is thereby obviated of cutting notches of the exact width ina fixed card to firmly and accurately hold nonpretensioned springs ofslightly varying thickness. The active or movable springs 6 and I areengaged by notches I3 in a movable card 14, actuatable by armature 3.Each of these movable contact springs B and I is bent or preformed so asto be pretensioned to a desired degree when installed in the relay. Thesprings B which are arranged to make contact with their mating springs 5upon actuation of the relay, are bent so as to exert a downward pressureagainst their respective notches I3 in card I4, and are normally heldout of contact with their mating springs by the reactive pressure ofsaid card M. Springs I, which are arranged to break contact with theirmating springs 5 upon actuation of the relay, are bent so as to normallyexert an upward pressure against their respective mating springs 5.

Upon actuation of the relay and the resulting downward movement of cardI4, the upward pressure of the notches I3 against the springs 6 isreleased and said springs 6 move downwardly solely as a result of theirresilient tension to engage their mating springs 5, and springs I areforced downwardly by card I4 so as to break contact with theirrespective mating springs 5. When the relay is not operated, thearmature 3 is held in its released position as shown in Figs. 1 and 2 bythe upwardly exerted resultant pressure of the downwardly tensionedcontact springs 6, the downwardly tensioned balancing springs I5 and theupwardly tensioned balancing springs It.

In accordance with the invention, the contact springs 5, 6 and I and thebalancing springs I5 and I6 are each bent transversely at two points intheir length (see Figs. 3 and 4) to a proper degree to provide thedesired amount of pretensioning when they are installed in the relay.When the individual springs are clamped in the spring pile-up assemblyI0 and placed in their normal tensioned position under the action ofspcolhead I2, card I4, or their mating springs as previously described,each of the individual springs lies in approximately a straight line. Inother words, when the relay is unoperated and thus the individual springis in its normal tensioned position, the individual spring deviates butslightly from a plane surface. As may be seen in Fig. 1, in which theresultant shape of the spring has been greatly exaggerated for purposesof clarity, each segment of each spring assumes a slightly curved shape.

The necessity of preforming the individual springs so that uponsubsequent pretensioning they will lie approximately straight arisesfrom the requirement for clearance between adjacent springs to obviatethe danger of undesirable interspring electrical cont-act. Such dangerparticularly exists where adjacent springs are ten-' sioned in oppositedirections as is the case of the upper two pairs of mating contactsprings in Fig. 1. The applicant has found that if but a single sharpbend is placed in each spring it is difiicult to reconcile the opposingfactors of interspring clearance, relay compactness, spring thickness,and required amount of spring tensioning. Further, when mating springsare pretensioned in opposite directions, the contacting ends of springshaving but a single bend are not sufiiciently parallel one to the otherto secure full use of the contacting surfaces.

* In Figs. 3 and 4 a contact spring II comprising a terminal portion I8,a clamping surface I9 and a contact means 20 is bent transversely beyondthe elastic limit of the material at two points 2I along its length. Itis to be understood that additional bends may be placed at other pointsthroughout the length either to position the operating length of thespring in relation to other parts for purposes other than pretensioning,or to more closely approximate a desired curve for purposes ofpretensioning.

In order to meet the several requirements for springs to be used inapproximate parallelism with other springs and formed to provide adesired degree of force, and particularly to meet the requirement forcontact springs to be used in electromagnetic devices in which suchsprings must be mounted in parallel proximity for reasons of relaycompactness and yet must have a certain minimum clearance betweenadjacent springs to prevent the completion of undesirable circuits, thesprings should be curved in such a manner that when flexed to theirnormal pretensioned position, they will lie approximately in a straightline, as was briefly mentioned above in several instances.

In the embodiment of the invention presented herein, the spring has beenbent to closely approximate the curve of a cantilever beam loaded at oneend, the equation of such an elastic curve being representable by:

3X2 X3 LA arm) in which Y the ordinate as measured from a horizontalplane through the clamping surface,

A=the maximum deflection of the spring as dc termined to provide a givencontact force,

X=the abscissa as measured from a vertical plane through the clampingpoint, and

L=the length of the spring as measured from the clamping point to thecenter line of contacts.

In the example of Fig. 5 the dotted line represents such an elasticcurve in which the length of the spring from clamping point to contacthas been selected as 2.077 inches and in which the characteristics ofthe metal and the amount of pretensioning required have necessitatedthat the maximum deflection A be 0.294 inch. If the spring wereconformed to the shape of the ideal elastic curve shown, it would form astraight line when the contact end was returned to the position of zerodeflection.

The heavy line in the graph of Fig. 5 represents a contact spring shapedto closely approximate the theoretical curve of a cantilever beam loadedat one end by the placing of a 6- degree 10-minute transverse bend 0.250inch from the clamping point, and an additional 5- degree transversebend at a point 1.000 inch from the clamping point. As is evident in thegraph of Fig. 5, the maximum departure from the ideal curve is 0.006inch at the first bend. If the re- .5 sultant pressure required isgreater than can be obtained without undue departure from thetheoretical curve, three .or more bends per spring maybe used.

"In order .to preform aspring :having two bends as shown in Figs. .3 and4, a punch and die assembly as depicted in Fig. 6 maybe used. The toolcomprises a male portion 22 and a female portion .23 between which:thesprings are pressed.

Locating pins 24 and .25 may :be provided to position the springs 26relative to the die '23. 'In order to concentrate the pressure on thesprings in the region where the bending takes place to provide greateruniformity in the amount of bend retained by the springs, the angles onthe male or punch portion are made sli htly sharper than thecorresponding angles on the female Or die portion. In the embodiment ofthe invention presented, for example, the angle A is one degree sharperthan the angle B, and the angle C is one degree sharper than its matingangle D. The applicant has found through extensive study of the bentsprings formed through the employment of this punch and die that thereis no appreciable grain distortion, reduction in spring thickness, orreduction in the strength .of the material .in the area adjacent thebend.

As many possible embodiments may be made of the above invention .and asmany changes might be made in the embodiment above set forth, it is tobe understood that all matter hereinbefore disclosedor shown in theaccompanying drawing is to be interpreted as illustrative and not in alimiting sense. Specifically, the applicant has considered theemployment of armature support or balancing springs of a material otherthan metal, the preforming of round wire springs in accordance with theinvention, the bending of the spring to approximate a curve other thanthat disclosed herein, and the application of the invention to thepreforming of springs for use in apparatus or mechanisms other thanelectromagnetic devices.

What is claimed is:

1. In a switch, a first and a second contact spring mounted in spacedproximity one to the other, each of said springs being shaped toapproximate a desired elastic curve to provide a desired degree ofsubsequent pretensioning, each of said springs comprising an elongatedmetallic cantilever fixed at one end only, a finite number greater thanone of permanent, transverse bends in the same direction in saidcantilever, and electrical contact means at the free end of saidcantilever, and means for causing movement of said first spring relativeto said second spring to move said first spring into operative positionrelative to said second spring.

2. A cantilever spring comprising an elongated metallic number, and aplurality of spaced apart permanent transverse bends in the samedirection in said member, the portion of said spring intermediate anadjacent pair of said bends being straight, and single unitary supportmeans fixing one end only of said spring and adapted to prevent movementof any part of said fixed end responsive to the application of anunbalanced force at any other point along said spring, the shape of saidspring approximating when not under load the shape it would assume if itwere under load but had no permanent transverse bends.

3. In a switch, a first and a second contact spring mounted in spacedproximity one to the other, each of said springs being shaped toapproximate the elastic curve of a cantilever beam 6 loaded at one endto provide a desired degree of subsequent pretensioning, eachof saidsprings comprising an elongated metallic cantilever fixed at one endonly, a finite number greater'than one of permanent transverse bends inthe same diloaded at one end to provide a desired degree of subsequentpretensioning, each of said springs comprising an elongated metalliccantilever fixed at one end only, only two permanent transverse 'bendsin the same direction in said cantilever, and electrical contact meansadjacent the free end of said cantilever, and means for causing movementof said first spring relative to said second spring to move said firstspring into operative position relative to said second spring.

5. In a switch, a first and a second contact spring mounted in spacedproximity one to the other, each of said springs being fixed at one endonly and shaped to approximate the elastic curve of a cantilever beamloaded at one end in order :to provide a desired degree of subsequentpretensioning, each of said springs comprising an elongated metallicmember, two permanent transverse bends in the same direction in saidmember, and electrical contact means adjacent the free end of saidmember, said elastic curve being representable by the equation in whichY is the ordinate as measured from a horizontal plane through one end ofthe spring, A is the maximum deflection of the spring as determined togive a given contact force, X is the abscissa as measured from avertical plane through one end of the spring, and L is the length of thespring.

6. A cantilever spring comprising an elongated metallic member, and onlytwo permanent transverse bends in the same direction in said member andsingle unitary support means fixing one end only of said spring andadapted to prevent movement of any part of said fixed end responsive tothe application of an unbalanced force at any other point along saidspring, the portion of said spring intermediate said two bends beingstraight, the shape of said spring approximating when not under load theshape it would assume if it were under load but had no permanenttransverse bends.

7. A spring shaped to approximate a certain elastic curve in order toprovide a desired degree of subsequent pretensioning comprising anelongated metallic member and a finite number greater than one permanenttransverse bend in the same direction in said member, said certainelastic curve being representable by the equation 3X X Y A 2L "$9 inwhich the ordinate Y is a distance measured from a horizontal planethrough one end of the spring, A is the maximum deflection of the springas determined to give a given reactive force, the abscissa X is adistance measured from a vertical plane through one end of the spring,and L is the length of the spring.

8. A cantilever contact spring comprising an elongated metallic memberand a plurality of spaced apart permanent transverse bends in the samedirection in said member, the portion of said spring intermediate anadjacent pair of said bends being straight, said bends being so locatedand of such degree that said spring is approximately straight when undera preselected load, and single unitary support means fixing one end onlyof said spring and adapted to prevent movement of any part of said fixedend responsive to the application of an unbalanced force at any otherpoint along said spring.

9. A cantilever contact spring approximating a straight line when undera preselected load comprising an elongatedmetallic member and apluralityof spaced apart permanent transverse bends in the samedirection in said member, the portion of said spring intermediate anadjacent pair of said bends being straight, and single unitary supportmeans fixing one end only of said spring and adapted to prevent movementof any part of said fixed end responsive to the application of anunbalanced force at any other point along said spring.

10. A cantilever contact spring approximating a straight line when.under a preselected load comprising an elongated metallic member and aplurality of spaced apart permanent transverse bends in the samedirection in said member, the

portion of said spring intermediate an adjacent pair of said bends beingstraight, said bends being of such degree that the shape of said springapproximates when not under load the shape it would assume if it wereunder said preselected load but had no permanent transverse bends, andsingle unitary support means fixing one end only of said spring andadapted to prevent movement of any part of said fixed end responsive tothe application of an unbalanced force at any other point along saidspring.

DONALD C. KOEHLER.

REFERENCES CITED The following references are of record in the file ofthis patent: s

UNITED STATES PATENTS Number Name Date 1,166,615 Mason et al. Jan. 4,1916 2,091,195 Dennebaum Aug. 24, 1937 2,112,148 Drolet Mar. 22, 19382,155,073 Ziska Apr. 18, 1939 2,156,465 I Vigren et a1. May 2, 19392,282,687 Vigren et a1 May 12,1942 2,301,547 Karbus Nov. 10, 19422,345,270 Lavoie Mar. 28, 1944 OTHER REFERENCES Machinerys Handbook(10th edition), edited by the Industrial Press. New York.

